The present disclosure is generally related to phase change ink, particularly phase change ink including an amorphous compound; a crystalline compound; an optional synergist; an optional dispersant; and a white colorant having a volume average particle size of from about 25 nanometers to less than 200 nanometers.
Phase change inks for color printing typically comprise a phase change ink carrier composition which is combined with a phase change ink compatible colorant. In a specific embodiment, a series of colored phase change inks can be formed by combining ink carrier compositions with compatible subtractive primary colorants. The subtractive primary colored phase change inks can comprise four component dyes, namely, cyan, magenta, yellow and black (CMYK), although the inks are not limited to these four colors. These subtractive primary colored inks can be formed by using a single dye or a mixture of dyes. For example, magenta can be obtained by using a mixture of Solvent Red Dyes or a composite black can be obtained by mixing several dyes. U.S. Pat. Nos. 4,889,560, 4,889,761, and U.S. Pat. No. 5,372,852, the disclosures of each of which are totally incorporated herein by reference, teach that the subtractive primary colorants employed can comprise dyes from the classes of Color Index (C.I.) Solvent Dyes, Disperse Dyes, modified Acid and Direct Dyes, and Basic Dyes. The colorants can also include pigments, as disclosed in, for example, U.S. Pat. No. 5,221,335, the disclosure of which is totally incorporated herein by reference. U.S. Pat. No. 5,621,022, the disclosure of which is totally incorporated herein by reference, discloses the use of a specific class of polymeric dyes in phase change ink compositions.
In addition to the CMYK color set, there is a need for white phase change ink. White inks are highly desired for certain graphic and labeling applications and for packaging applications. They are most typically used on transparent and dark substrates to compliment or provide a background to text or graphics. White text on smoke colored plastic is particularly striking. White colorants are pigments, which can comprise a variety of materials, most typically titanium dioxide. A general description of titanium dioxide properties can be found, for example, in Kirk-Othmer Encyclopedia of Chemical Technology, 4th Ed., (John Wiley & Sons, NY), Vol. 24, pages 239-240, which is hereby incorporated by reference herein. Because white is produced by light scattering, a pigment having a volume average particle size of from about 200 to about 400 nanometers is generally selected to provide sufficient opacity. As the particle size in a dispersion decreases below this range, the dispersion selectively scatters blue light and appears bluish white. As the particle size decreases further, it will not scatter visible light and will become transparent. For example, smaller nanometer sized titanium dioxide pigments are known, but they are colorless. White pigments pose a particular challenge for formulating into ink jet inks owing to their very high density. TiO2 has a specific gravity of about 4. One problem is that the white pigment tends to settle in molten ink. Recirculating print heads have been required to address the problem of white pigment settling in molten ink. A combination of dense dispersions, for example, from about 2 to about 4 grams of pigment per milliliter of pigment and dispersion, relatively large size, and high loadings of pigment make it particularly challenging to design a storage stable and jettable white curable solid ink.
While known compositions and processes are suitable for their intended purposes, a need remains for phase change white inks that are suitable for ink jet printing processes, such as piezoelectric ink jet processes and the like. In addition, a need remains for phase change white inks that are suitable for acoustic ink jet printing processes. Further, a need remains for improved digital ink jet phase change white inks that can provide excellent print quality on colored package substrates including colored cardboard or thick paper, particularly brown or other darker colors, that can provide the desirable esthetic appearance and appeal of white prints over dark substrates, that provide improved storage stability and low Newtonian viscosity. Further, a need remains for improved digital ink jet phase change white inks that can provide acceptable rub and scratch resistance, especially for packaging applications where package handling increases the possibility of rub and scratch damage. Further, a need remains for stable, robust white phase change ink that can be used with phase change ink jet print heads without requiring recirculating or other complex methods, that are resistant to settling in the molten state, and that are sufficiently opaque to provide adequate contrast of the printed image.
The appropriate components and process aspects of the each of the foregoing U.S. patents and patent Publications may be selected for the present disclosure in embodiments thereof. Further, throughout this application, various publications, patents, and published patent applications are referred to by an identifying citation. The disclosures of the publications, patents, and published patent applications referenced in this application are hereby incorporated by reference into the present disclosure to more fully describe the state of the art to which this invention pertains.